Introduction: 15600mA Power Bank @ Just Rs.370 ($6)
Recently I have ordered a Power Bank Kit from Hong Kong (China) costing just Rs. 370 ($6). The Kit arrived only with the cabinet and PCB. The required 18650 cells were not included. In market 18650 cell costs around Rs.150 ($2) each. Six of such cell would have made a hole in my pocket costing Rs.900 ($13).
To save the cell cost, I have arranged an old laptop battery and disassembled it. All laptop batteries are made of at least 4 to 6 18650 cells. I tested them and they were giving 3.67 to 3.72 volt, even after the battery was not used since 2 years. Generally, 18650 cells are rated for 2600mA it means 15600mA for 6 cells. Following steps describes how to assemble the Power Bank.
Step 1: The Package
The package I have received from Hong Kong containing PCB and plastic cabinet.
Step 2: Old Laptop Battery
CAREFULLY disassemble the laptop battery with a flat screw driver. Try not to puncture the cells inside the battery case, it may start fire due to short circuit. You may have heard about battery fire and explosion cases in Mobile phones. Remove the charging/discharging circuit inside the battery and remove the 18650 cells. The cell are connected in series and parallel (both). Cut the connecting wires and strips and separate each cell. Check the voltage of each cell. Remove the cells where are giving less than 3.5 volt.
Step 3: 18650 Cells Removed From Laptop Battery
These are the cells recovered from old laptop battery. All the 18650 cells has a ring on one side which indicate +ve side. Unlike generally available AA and AAA cells which has a clear +ve terminal protruding over the cell body, 18650 cells has flat surface on both terminals.
Remove all the connecting wires and strips from cell's top and bottom terminals and make them clear.
Step 4: Prepare Connecting Wire
We need to connect all the 6 cells in parallel. Prepare two copper strips or flatten two twisted braided wire pieces.
Step 5: Prepare the Cells for Soldering
Pre-solder the cells by applying solder on both the terminals. Be careful, do not melt the plastic insulation, specially from the +ve terminal.
Step 6: Connect the Cells in Parallel
Connect all the cells in parallel. To solder, hold all top +ve sides on a clamp (I clamped them using two thick books) and start soldering the copper strip on each cell's terminal. Repeat the same process for -ve terminal.
Step 7: Battery Pack
This is the final battery pack you will get. All the +ve and -ve terminals are now connected through a copper wire each. Paste a proper insulating tape on both the sides.
Step 8: Install the Power Bank Kit PCB
Install the PCB in the plastic case carefully. You may need some trimming/filing of PCB and case.
Step 9: Install Battery Pack
Carefully install the battery pack inside the cabinet. Do not apply excessive pressure as the connection between the cell and copper wire may break. Cut the excess copper wire. The PCB has a mark B+ and B- which indicate +ve and -ve terminal connections to the battery. Solder the battery pack's +ve to B+ and -ve to B-.
Step 10: Install Button and LED Indicator Riser
Install provided button and acrylic LED indicator riser at given place, inside the power bank case. I have also glued them so that they will remain at their place when closing the case.
Step 11: Test the Power Bank
Finally test the power bank by connecting a mobile or any other device. The LED will lit up showing how much charge is left. There are 4 level indicators (25% each). Connect a mobile charger to power bank and check whether the LEDs blinking or not. Hold the button for 5 seconds and the White Torch LED will lit up. Hold the button for another 5 seconds and it will be off. (your kit may not come with torch function)
If every thing is working fine, close the cabinet/case. Now you can charge 4 devices at a time. Enjoy.
Step 12: !!! SAFETY !!!
As old/used 18650 cells are used in this project, the cells should be balanced with the others, or you run a higher risk of failure. It will be better if you use brand new 18650, although it will increase the project cost and spoil the charm. I have not included any information regarding cell balancing. Alternative to 18650 are normal AA NiMH batteries.